The radial tire is excellent in the wear resistance and steering stability because cords in a carcass ply are arranged to extend substantially in a direction perpendicular to an equatorial plane of the tire. Therefore, radial tires are frequently used as compared with bias tires in not only passenger cars but also vehicles such as small-size truck, truck and bus with a recent advance of the high-speed performance of those vehicles.
However, the high-speed running of the vehicles is daily carried out in accordance with the improvement and expansion of road systems and the like. For this end, it is strongly demanded to sufficiently control the wandering phenomenon liable to be caused in the radial tire rather than the bias tire to thereby enhance the straight running stability and more improve the safety.
Viewing the occurrence of wandering phenomenon in the radial tire, as shown in FIG. 1, when the tire T is rotated on a slant face S such as rut or the like, each of load W, reaction force F.sub.R from road surface and camber thrust F.sub.C is applied to the tire T and hence lateral force F.sub.Y as a resultant force of horizontal components of these forces acts thereto. The tire of radial structure is high in the treading rigidity as compared with the tire of bias structure, and also the rigidity of the tread portion is fairly higher than that of the side portion due to the structure of the radial tire. The camber thrust F.sub.C is made smaller than that of the tire of bias structure and hence lateral force F.sub.Y directing to a lower side of the slant face S becomes relatively large by a quantity corresponding to the reduced quantity of the camber thrust F.sub.C. As a result, the tire T strongly tends to slip downward on the slant face and it is difficult to ride over a rut and hence the wandering phenomenon is caused.
As shown at radial section of the tire in FIG. 2, when the tire is rotated on the slant face S, the camber thrust FC is caused based on the fact that the tread portion T.sub.r tends to strongly contact with ground at an upper side or a mountain side of the slant face S and rise upward at a valley side of the slant face S, and particularly falling-down deformation of a region Bu in the vicinity of the buttress of the tire side portion or so-called bulging deformation bside toward the mountain side is caused at the mountain side due to the strong ground contact of the tread portion T.sub.r and such a bulging deformation bside brings about bending deformation of the tread portion T.sub.r near to a ground contact end or so-called pushing-out deformation b.sub.sho, and such a pushing-out deformation b.sub.sho in a zone near to the ground contact end further gives shearing deformation S.sub.S as shown by dotted lines to the tread rubber in a portion near to the ground contact end, and such a shearing deformation creates a lateral force FCS directing to the upper side of the slant face S.
As regards the camber thrust F.sub.C, in the bias tire having a small difference of rigidity among constitutional parts of the tire and a relatively soft structure, sufficiently large bulging deformation b.sub.side and pushing-out deformation bsho can be insured and hence the camber thrust F.sub.C created becomes large. Since the rigidity of the tread portion in the radial tire is considerably higher than the rigidity of the side portion, a tendency of creating escape deformation in a direction opposite to the above as exaggeratedly shown by a phantom line becomes strong in the tire side portion and hence the shearing deformation SS of the tread rubber becomes naturally small and it is obliged to reduce the camber thrust F.sub.C.
In order to increase the camber thrust in the pneumatic radial tire, therefore, it is effective that the bulging deformation bside of the region Bu is made large and the transmission efficiency of the bulging deformation b.sub.side is enhanced to make large the pushing-out deformation b.sub.sho in the zone near to the ground contact end to thereby increase the shearing deformation S.sub.S of the tread rubber at the ground contact end portion. It is effective that the ground contact area of the tire to the slant face S is made large to increase a total amount of lateral force FCS directing to the upper side of the slant face S.
It is, therefore, an object of the invention to provide pneumatic radial tires sufficiently controlling the occurrence of wandering phenomenon on the slant face of the rut or the like to largely improve the straight running stability by increasing the camber thrust of the radial tire without degrading the properties inherent to the radial tire.